Hydrogeochemical Processes Influencing Groundwater Quality Within The Lower Pra Basin, Ghana

ABSTRACT

Hydrogeochemical and social impact studies were carried out within the Lower Pra Basin where groundwater serves as a source of potable water supply to majority of the communities. The main objective of the study was to investigate the hydrogeochemical processes and the anthropogenic impact that influence groundwater as well as the perception of inhabitants about the impact of their socio-economic activities on the quality of groundwater and subsequently make recommendations towards proper management and development of groundwater resources within the basin. The methodology involved quarterly sampling of selected surface and groundwater sources between January 2011 and October 2012 for major ions, minor ions, stable isotopes of deuterium (2H) and oxygen-18 (18O) and trace metals analyses as well as administration of questionnaires designed to collect information on the socio-economic impact on the water resources within the basin. In all, a chemical data-base on three hundred and ninety seven (397) point sources was generated and three hundred (300) questionnaires were administered. The hydrochemical results show that, the major processes responsible for chemical evolution of groundwater include: silicate (SiO4)4- weathering, ion-exchange reactions, sea aerosol spray, the leaching of biotite, chlorite and actinolite. The groundwater is mildly acidic to neutral (pH 3.5 – 7.3) due principally to natural biogeochemical processes. Groundwater acidity studies show that, notwithstanding the moderately low pH, the groundwater still has the potential to neutralize acids due largely to the presence of silicates/aluminosilicates. Results of the Total Dissolved Solids (TDS) show that 98.6 % of groundwater is fresh (TDS < 500 mg/L). The relative abundance of cations and anions is in the order: Na+ > Ca2+ > Mg2+ > K+ and HCO3- > Cl- > SO42- respectively. Stable isotopes results show that, the groundwater emanated primarily from meteoric origin with evaporation playing an insignificant role on the infiltrating water. However, with reference to the Local Meteoric Water Line (LMWL) for the Accra Plains by Akiti (1986),

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the results suggest evidence of isotopic enrichment by evaporation on the surface or in the unsaturated zone before recharge into the groundwater system. Hydrochemical facies delineated two main water types, the Ca-Mg-HCO3 and Na-HCO3 water types, with Ca-Mg-Cl and Na-Cl as minor water types. Using Q-mode Hierarchical Cluster Analyses (HCA), surface and groundwater within the basin have been characterized into four (4) water groups and five (5) subgroups. Water quality data for subgroups show that, Groups 1 and 2 waters both represent a transition zone between Ca-Mg-HCO3/Na-HCO3 and Na-Cl/ Ca-Mg-Cl /Na-SO4 water types and therefore, can be regarded as transition zones between groundwaters which evolved from Ca-Mg-HCO3 water type into Na-HCO3, Ca-Mg-Cl, and Na-Cl water types along its flow-path. Hydrochemical data also suggest that, groundwater within the basin is primarily undergoing recharge processes involving freshwater mixing with geochemically different ionic signatures than processes involving saline-freshwater mixing. PCA using Varimax with Kaiser Normalization for component matrix has delineated three main processes; i.e. natural geochemical and biochemical processes (water-soil-rock interactions), incongruent dissolution of silicates/aluminosilicates, and pollution of the water resources principally from agricultural inputs. The trace metal results show that, groundwater in some communities within the basin is contaminated, due to natural and anthropogenic sources with Al (19.2 % of boreholes), Se (18.4 % of boreholes), Cd (18.0 % of boreholes), As (11.6 % of boreholes), Pb (39.6 % of boreholes), Mn (5.6 % of boreholes), Hg (42.0 % of boreholes) and Fe (21.6 % of boreholes) concentrations exceeding the WHO (2004) guideline limits for drinking water. Results from the social impact survey shows that, majority (93.0 %) of the inhabitants depend on groundwater. The survey also shows that, 58.0 % of the inhabitants use groundwater for their drinking water needs due largely to quality reasons. With respect to the cost of abstracting groundwater, 68.0 % of the inhabitants are of the view that, the cost is appreciably low. Results further shows that, 13.3 % of

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groundwater within the basin may become contaminated through solid waste disposal, while, 2.0 % of groundwater may become contaminated through human waste excreta. Regarding contamination of groundwater through farming and mining activities, surface waters which are most often used in these activities are endangered since 81.0 % of farmers spray their farms with agrochemicals and all mining activities especially “galamsey”are located close to surface water course.